Polyolefins generally have excellent mechanical properties, so that they are used in many fields such as fields of various molded products. With the diversification of property requirements for the polyolefins, polyolefins of various properties have been desired in recent years, and besides increase of productivity of the polyolefins has been also desired.
As catalysts for producing polyolefins, titanium catalysts comprising a titanium compound and an organoaluminum compound and vanadium catalysts comprising a vanadium compound and an organoaluminum compound have been hitherto known.
In addition, Ziegler catalysts comprising a metallocene compound such as zirconocene and an organoaluminum oxy-compound (aluminoxane) are known as catalysts capable of producing polyolefins with a high polymerization activity.
Recently, a transition metal compound having a ligand of diimine structure has been proposed as a novel olefin polymerization catalyst (see International Patent Publication No. 9623010).
As a novel olefin polymerization catalyst, further, a transition metal compound having a salicylaldoimine ligand has been recently proposed by the present applicant in Japanese Patent Application No. 132706/1998. This catalyst exhibits high olefin polymerization activity at a temperature of about 25° C. In the production of polyolefins using an industrial process, however, a polymerization catalyst exhibiting higher polymerization activity at high temperatures is desired because higher temperatures are advantageous in that the reaction heat can be easily removed and the viscosity of the reaction solution can be lowered to thereby enhance productivity.